Add image decryption and inline preview for WeChat V2 format

Support all three .dat encryption formats:
- Old XOR format: single-byte XOR, auto-detect key from magic bytes
- V1 format: AES-ECB with fixed key (md5("0")[:16]) + XOR tail
- V2 format (2025-08+): AES-128-ECB + raw middle + XOR tail

New files:
- decode_image.py: unified image decryption module (XOR/V1/V2)
- find_image_key.py: extract AES key from WeChat process memory
- find_image_key_monitor.py: continuous monitoring version for key capture

monitor_web.py changes:
- Inline image preview in Web UI with async decryption
- MonitorDBCache for mtime-based DB decryption caching
- username-to-DB mapping for image resolution chain
- /img/ endpoint for serving decoded images
- SSE image_update events for real-time preview updates

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -3,6 +3,7 @@ all_keys.json
 wechat_key.txt
 config.json
 decrypted/
+decoded_images/
 *.db
 *.db-shm
 *.db-wal